Name:_________________________
Density, Buoyancy, and Force Diagrams PhET Lab)
http://phet.colorado.edu/en/simulation/density
Introduction
You’ve heard that oil floats on water. You also know that ice cubes normally float in a
glass of water. Why? What causes some things to float in water (corks, ice, dogs) and
some to sink in water (rocks, metal spoons, mobsters)? Density is often described as the
amount of mass crammed into a volume, and is illustrated by the formula shown below.
The units for density are expressed g/cm3, g/mL, kg/m3, and kg/L. In this simulation, we
will use kg/L. Water’s density is 1.00 kg/L.
Important Formulas:
  mV


W fluid  FBuoyant


Wobject  mg
Procedure: PhET Simulations  Play With Sims  Physics  Density
 Take a few minutes and familiarize yourself with the simulation before moving on.
Free Body Diagrams for Floating Objects:
 Using the simulator, grab the various blocks, lift them over the water and drop into the water a few times.
The %-Density Ratio Trick
 Estimate the percentage of the wood, ice, and Styrofoam block that is under water while those blocks are
floating. Remember: the density of water is 1.00 kg/L


% Wood under water
% Ice under water
% Styrofoam under water
Density of Wood
Density of Ice
Density of Styrofoam
Compare your estimations to the densities shown on the chart
. Place those densities in the chart
above.
Create a formula to show how much of a floating object would be submerged in any fluid.
Calculating Density
Using the simulation, select each scenario listed on the right, complete the tables below.
Same Mass
Block
% submerged
Mass, kg
Blue
5.00 kg
Yellow
5.00 kg
Green
5.00 kg
Red
Sunk / 100%
5.00 kg
Volume, L
Density, kg/L
Same Volume
Block
% submerged
Mass, kg
Volume, L
Density, kg/L
% submerged
Mass, kg
Volume, L
Density, kg/L
% submerged
Mass, kg
Volume, L
Density, kg/L
Blue
Yellow
Green
Red
Same Density
Block
Blue
Yellow
Green
Red
Mystery
Block
Blue, B
Yellow, A
Green, C
Red, D
Purple, E
Open up the “Show Table” and identify the mystery blocks:
A=
,B=
,C=
,D=
,E=
Conclusion Questions
1. Increasing the size of an object increases / decreases / doesn’t change the object’s density (circle)
2. An object with a density of .67 kg/L would float 1/3 , 1/2 , 2/3 under water. (circle)
3. A floating object has an upward buoyant force that is equal to / larger than / less than the downward weight.
4. An ice cube dropped into a glass of 100% ethanol (density=. 789 kg/L) would sink / float.
5. Using the formula you found for the %-density ratio, determine the percentage of a wood block (ρ=.400 kg/L)
that would be submerged in ethanol.
__________________________
6. Determine the density of an unknown metal that displaces 4.5 L of water and is found to have a mass of 25.4
kg.
__________________________
7. How much water will a 1.00 kg metal block displace with a density of 7.00 kg/L?
____________________
8. How much water will a 1.00 kg plastic block (ρ= 0.60 kg/L) displace when floating? (careful)
________________________________________________________________________________________
9. The red block in the “Same Volume” floats in water. The blue block sinks in water. Using your data from the
chart above and your knowledge of buoyant forces and weights, what volume of the blue block
blue
would float above the water line if the blue block was placed on top of the red block in the
water? ____________________
red